by Michon Scott · design by Robert Simmon and Michon Scott · August 1, 2006

When it comes to heat waves, Stuart Gaffin considers himself pretty lucky. An associate research scientist with the Earth Institute at Columbia University, Gaffin frequently studies heat waves. So far, however, he has managed to avoid getting caught in a deadly one, such as the Chicago heat wave of 1995, or the Paris heat wave of 2003. He lives in New York City, and the worst heat wave he remembers struck when he was in a nearby rural area. “We had no air conditioning, and it was so bad, it was like 106 degrees Fahrenheit,” he recalls. He left the countryside and rushed back to the city where his air-conditioned apartment awaited.

He admits the move was a little ironic.

Ironic because Gaffin also studies what’s known as the urban heat island—the tendency of cities to experience warmer temperatures than surrounding rural and suburban areas. In leaving the countryside, he actually headed for a hotter environment. Gaffin’s New York City apartment has air conditioning, but “we have a lot of poor communities in New York with low air conditioning rates,” he says. And electricity blackouts caused by a surge in demand during heat waves can catch anyone. “The heat wave problem is a real one, and one of the primary reasons to look at urban heat islands,” he explains.

Two trends cause Gaffin to think seriously about urban heat islands. One trend is an increase in the number of city dwellers. “About half the world’s population—3 billion people—now live in cities. In a couple decades, it’s going to be 5 billion people,” he says. In developing countries, people often migrate to cities because of crop failures, natural disasters, or armed conflicts, not because cities have robust economies capable of supporting more people. In the coming decades, many new city dwellers will be desperately poor. With little access to air conditioning, refrigeration, or medical care, the world’s urban poor will be particularly vulnerable to heat wave health hazards.

The other trend that concerns Gaffin is climate change. He is no skeptic about global warming; he accepts the scientific community’s findings that Earth is heating up, pushed in part by human activity. But even if countries can reduce their greenhouse gas emissions, Gaffin suspects a certain amount of further warming is inevitable. “I think mitigating global warming is important,” he says, “but even with mitigation, all these people living in cities are going to experience some warming.” Based on research he conducted for the U.S. Global Change Research Program, he expects a warmer climate to worsen heat waves. “Right now, we average about 14 days each summer above 90 degrees [in New York]. In a couple decades, we could be experiencing 30 days or more,” he says.

“So we have two forces—urban heat islands and global warming—that are reinforcing each other and are going to create hot, hot conditions for more than half the world’s population,” Gaffin explains. “How do we make cities more habitable in the future?”

In 2002, Gaffin and several colleagues set out to answer that question for the United States’ most populated city: New York. To help urban planners figure out how to make city heat more bearable, Gaffin and his colleagues first had to consider how urban areas heat up their environments.

On May 11-12, 1997, NASA used a specially outfitted Lear Jet to collect thermal data on metropolitan Atlanta, Georgia. Nicknamed “Hot-Lanta” by some of its residents, the city saw daytime air temperatures of only about 26.7 degrees Celsius (80 degrees Fahrenheit) on those days, but some of its surface temperatures soared to 47.8 degrees Celsius (118 degrees Fahrenheit). In this image, blue shows cool temperatures and red shows warm temperatures. Pockets of especially hot temperatures appear in white. (Image courtesy NASA/Goddard Space Flight Center Scientific Visualization Studio.)